Pesticides are killing frogs

  Last July, Dr. Donald F. Anthrop wrote a letter, "Pesticides killing Frogs? Poppycock" (HCN, 7/7/03: Pesticides killing Frogs? Poppycock), criticizing an earlier report by Cosmo Garvin (HCN, 5/26/03) about possible effects of pesticides on frog populations. At the end of Dr. Anthrop’s letter, he stated: "This is a sorry excuse for scientific research."

I think it is important that the question of pesticide effects on wildlife be taken seriously, and I see lots of good evidence (such as research published in peer-review scientific journals) that Garvin’s story was based on standard and acceptable scientific research.

Dr. Anthrop’s first point was that "despite the fact that pesticide residues found in mountain frogs are far below lethal levels, the argument seems to be that since declines in frog populations have occurred in mountain regions downwind from agricultural regions, pesticides are responsible."

I find this statement to be misleading, because "lethal" typically means that individuals exposed to the chemical die in a few hours or days. Sub-lethal exposure levels are much more frequently observed than lethal levels, because pesticide applications are typically greatly diluted by air and water. The important ecological point is that even at very low sub-lethal exposure levels, solid scientific research has shown that pesticides adversely affect development, physiology, hormone activity, behavior, and reproduction of amphibians (including frogs). Sub-lethal pesticide effects can adversely affect wildlife populations, even if there is no increased mortality. For example, if male frogs can’t croak, they won’t attract mates, and the population will decline. Published results of careful scientific research by several ecotoxicologists show that, at very low (parts per billion) levels, many kinds of pesticides can interfere with normal development, functioning and reproduction of a variety of organisms, including frogs. A reasonable hypothesis (and area for research) is that, over generations of time, wildlife populations are declining due to sub-lethal adverse effects of pesticides. This kind of ordinary and acceptable science is being done, reviewed and published every day.

Dr. Anthrop’s second point was that "if pesticides constitute the culprit, why didn’t the frog populations decline long ago?" The major decline of many amphibian populations occurred in the 1960s or 1970s (you can check this out by "Googling" your favorite amphibians). Pesticides have been used in exponentially increasing amounts since the 1950s. To take a specific example, Atrazine, introduced in the 1960s, is the most commonly used herbicide in North America. Research by several scientists has shown that Atrazine adversely affects sexual development and behavior in frogs, at levels far below those that kill frogs outright. For pesticides in general, and in the specific case of Atrazine, there is a reasonable historical match between increasing pesticide use and decreasing amphibian populations. The historical pattern sets the stage for research to test hypotheses about whether specific sub-lethal mechanisms are in fact causing the decline. To me, this sounds like a reasonable scientific approach.

We may not have the whole story on which specific pesticides to implicate in the amphibian decline, but it seems reasonable to me, as a practicing ecologist and toxicologist, to be concerned about pesticide effects on wildlife.

Stanley Dodson
Madison, Wisconsin

The author, who became a naturalist in Grand Junction, Colorado, is a professor of zoology at the University of Wisconsin.